I definitely think any "every comment left by this user anywhere" variant should require opting in before someone's comments could be tracked that way, and I don't know where I stand on "every comment left by this user in this community" or "every post made by this user to this community" - I can see the utility, but those do feel awfully broad. On the other hand, "every comment left by this user on this entry" seems a lot narrower to me - someone could already get notifications for "every comment left on this entry", after all.